When an object is moved off its course, we say it has been deflected. The Coriolis effect describes how objects that are not connected to the ground seem to get deflected as they travel long distances around Earth.

The Coriolis effect is responsible for many large-scale weather patterns. The key to the Coriolis effect lies in the planet's rotation from west to east. Specifically, Earth rotates faster at the Equator than it does at the poles. Every point on Earth takes the same amount of time—24 hours—to make a complete rotation. Yet points along the equator have to travel a much longer distance than points near the poles and, therefore, move at a higher speed. Equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour, whereas near the poles the planet rotates at a slow 0.00008 kilometers (0.00005 miles) per hour. 

These different speeds of rotation are the reason for the Coriolis effect. Let's pretend you're standing at the Equator and want to throw a ball to your friend in the middle of North America. As you throw the ball, both you and the ball are already moving eastward more quickly than your friend is. So if you try to throw the ball in a straight line, it will land to the right of your friend. Remember, your friend in North America is moving east at a slower speed.

Now let's pretend you're standing at the North Pole. When you throw the ball to your friend, it will again appear to land to the right of him. This time, it's because he's moving faster than you are and has moved ahead of the ball. Everywhere you play this game of global catch, in the Northern Hemisphere the ball will deflect to the right. 

This apparent deflection is called the Coriolis effect. Fluids traveling across large areas, such as air currents, are affected in the same way as the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents bend to the left.

The strength of the Coriolis effect depends on velocity, or speed of travel in a particular direction. It depends both on the velocity of Earth and the velocity of the object or fluid being deflected. The higher the speed or the longer the distance, the stronger the Coriolis effect.  

Weather Patterns

Certain weather patterns, such as cyclones and trade winds, are due to the Coriolis effect. A key factor is air pressure, or the averaged weight of air molecules pressing down on Earth. Weather systems can be low-pressure or high-pressure, and air tends to move from high-pressure to low-pressure areas. 

Cyclones are low-pressure systems that suck air into their center, or eye. In the Northern Hemisphere, as fluids move from high-pressure systems to low-pressure systems, they are subject to the Coriolis effect. In other words, they aim for low-pressure systems, but pass them to the right. As air masses are pulled into cyclones from all directions, the storm system—a hurricane—seems to rotate counterclockwise. In the Southern Hemisphere, the opposite happens: currents are deflected to the left, and storm systems seem to rotate clockwise. 

Outside of storm systems, the Coriolis effect also helps define regular wind patterns around the globe. As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right, or east, as they move northward. Then the currents descend back toward the ground and slowly move from the northeast to the southwest, back toward the Equator. The constantly circulating patterns of these air masses are known as trade winds.

Impact on Human Activity

Airplanes and rockets are impacted by the Coriolis effect as well. The directions of prevailing winds are largely determined by this effect, so pilots must take it into account when charting flight paths over long distances. 

The Coriolis effect can also change bullet trajectories. Sometimes, military snipers have to consider the effect since even a small deflection could injure innocent people. 

The Coriolis Effect on Other Planets 

Earth rotates fairly slowly compared to other planets. The slow rotation of Earth means the Coriolis effect is not strong enough to be seen at slow speeds over short distances. Jupiter, on the other hand, has the fastest rotation in the solar system. On Jupiter, the Coriolis effect actually transforms north-south winds into east-west winds, some traveling more than 610 kilometers (380 miles) per hour. The divisions between winds that blow mostly to the east and those that blow mostly to the west create clear horizontal divisions, called belts, among the planet's clouds. The boundaries between these fast-moving belts are highly active storm regions. The 180-year-old Great Red Spot is perhaps the most famous of these storms.

The Coriolis Effect Closer to Home

There are some realistic ways of observing the Coriolis effect. Say you and a friend are throwing a ball back and forth while sitting on a merry-go-round. When the merry-go-round is not rotating, throwing the ball is easy. While the merry-go-round is rotating, however, the ball won't make it to your friend without significant force. Thrown with regular effort, the ball appears to curve to the right. Another friend who is standing off of the merry-go-round will be able to tell you what's really happening. The ball is moving straight through the air, but you and your friend are rotating and moving out of its path.

 

Coriolis Effect
Storms in the north swing counter-clockwise: the Coriolis effect. Storms in the south swing with the clock, and winds tend to pass to the left!
air current
Noun

flowing movement of air within a larger body of air.

Noun

a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

Noun

a dip or depression in the surface of the land or ocean floor.

belt
Noun

dark-colored band of clouds on Jupiter or Saturn.

Noun

line separating geographical areas.

breeze
Noun

light wind or air current.

Noun

type of map with information useful to ocean or air navigators.

civilian
Noun

person who is not in the military.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

Noun

the result of Earth's rotation on weather patterns and ocean currents. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere.

Coriolis force
Noun

force that explains the paths of objects on rotating bodies.

counter-clockwise
Adjective

circular motion to the left.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

cyclone
Noun

weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.

deflect
Verb

to alter from a straight line.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

fluid
Noun

material that is able to flow and change shape.

force
Noun

power or energy that activates movement.

Great Red Spot
Noun

enormous storm in Jupiter's Southern Hemisphere, which has been observed for more than 100 years.

horizontal
Adjective

left-right direction or parallel to the Earth and the horizon.

hurricane
Noun

tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.

infrastructure
Noun

structures and facilities necessary for the functioning of a society, such as roads.

Jupiter
Noun

largest planet in the solar system, the fifth planet from the Sun.

low-pressure system
Noun

weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.

manufacture
Verb

to make or produce a good, usually for sale.

Noun

armed forces.

Northern Hemisphere
Noun

half of the Earth between the North Pole and the Equator.

Noun

fixed point that, along with the South Pole, forms the axis on which the Earth spins.

observe
Verb

to watch.

Noun

large, spherical celestial body that regularly rotates around a star.

pole
Noun

extreme north or south point of the Earth's axis.

precise
Adjective

exact.

prevailing wind
Noun

wind that blows from one direction.

Noun

object's complete turn around its own axis.

satellite imagery
Noun

photographs of a planet taken by or from a satellite.

significant
Adjective

important or impressive.

sniper
Noun

gunman who fires from a concealed place.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

Southern Hemisphere
Noun

half of the Earth between the South Pole and the Equator.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

trade wind
Noun

winds that blow toward the Equator, from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.

trajectory
Noun
path of an object moving in space under the influence of such forces as thrust, wind resistance, and gravity.
transform
Verb

to change in appearance or purpose.

urban legend
Noun

modern myth or piece of folklore.

velocity
Noun

measurement of the rate and direction of change in the position of an object.

weather pattern
Noun

repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.